Harmonic gyrotrons operating in high-order symmetric modes

It is shown that gyrotrons operating at cyclotron harmonics can be designed for operation in symmetric TE0,p-modes. Such operation in fundamental harmonic gyrotrons is possible only at small radial indices ( p≤3) because of the severe mode competition with TE2,p-modes, which are equally coupled to annular beams as the symmetric modes. At cyclotron harmonics, however, this “degeneracy” of coupling is absent, and there is a region in the parameter space where harmonic gyrotrons can steadily operate in symmetric modes. This fact is especially important for sub-THz and THz-range gyrotrons where ohmic losses limit the power achievable in continuous-wave and high duty cycle regimes.